The present invention relates to a method of controlling combustion in a furnace of a boiler or the like, and more particularly, to a method of controlling combustion in a furnace of a plant required to reduce the amount of nitrogen oxides (NO.sub.x) generated therein.
The formation of nitrogen oxides (NO.sub.x) is one of serious problems which must be taken into consideration particularly in combustion in a boiler employed in a thermal power plant or the like. The combustion conditions are increasingly stringent partly because regulatory standards are set for limiting the extent of NO.sub.x production from boilers. Accordingly, various techniques are being developed to control the formation of NO.sub.x by modifying the combustion method. Particularly, when pulverized coal is burned, there are large variations in amount of generation of NO.sub.x depending on the type of coal as compared with the combustion of other fuels; hence, it is important to develop a method of controlling combustion which makes it possible to limit the amount of NO.sub.x emission. However, it is conventionally difficult to control the NO.sub.x emission, since the formation of NO.sub.x is a very complex phenomena involving aerodynamics, physical, chemical and thermal considerations.
One proposal of the prior art to control the NO.sub.x emission is an improvement in the structure of a furnace, e.g., U.S. Pat. No. 4,294,178 "Tangential Firing System" (Oct. 13, '81). Described therein is a steam generator which is arranged such that the pulverized coal and primary air introduced from each of the four corners of a furnace are directed tangentially to an imaginary circle in the center of the furnace so as to minimize both the formation of waterwall slagging and corrosion and also the formation of nitrogen oxides, and which includes means for introducing the secondary air so that the air is directed tangentially to a second imaginary circle.
Further, as prior art concerning the burner structure, there is U.S. Pat. No. 4,173,118 directed to "Fuel Combustion Apparatus Employing Staged Combustion" (Nov. 6, '79). Described therein is an apparatus having a combustor with a double concentric combustion cylinder for effecting combustion in each of the rich mixture, lean mixture and dilution zones.
However, there is no prior art known concerning a technique to effect an on-line control of the amount of NO.sub.x produced in a furnace. One of the reasons for this is that there is no known technique to properly know and understand the state of NO.sub.x during combustion. In other words, even in a plant provided with a burner for reducing NO.sub.x, the state of formation of NO.sub.x during combustion is not properly understood; hence, there are no information and instruction available for controlling the amounts of fuel and air supplied. As the prior art having improved the response to changes in load demand of boilers, there is U.S. Pat. No. 4,332,207 "Method of Improving Load Response on Coal-Fired Boilers" (June 1, '82). However, there is no prior art about a technique to effect on-line control of the NO.sub.x emission in such a plant that there are changes in properties of fuel, e.g., the change in type of coal.